The extraction of gold and silver from polymetallic ores using halogens in brine (NaCl/NaBr in water) has been reported (U.S. Pat. No. 7,537,741).
A closed loop method for extracting precious metals from an ore, using halogens has also been developed, where the halogens (chlorine and bromine) are used as free halogens at the time of extracting the precious metals and are recycled by electrolysis of the barren solution in the form of hypohalides, i.e. sodium hypochlorites NaOCl and sodium hypobromites NaOBr (US Patent application 2013/0074655). The free halogens are liberated from the hypohalide state by operating the leaching under acidic conditions. The following equation summarizes the main steps of this closed loop method, which is illustrated in FIG. 1.
Liberation of free halogens from hypohalides2OCl−/2OBr−+2H+→Cl2/Br2+H2O
Gold extraction with halogens mixture3Br2+Ore(Au)→2AuBr3+Ore(depleted)3AuBr3+3Cl2→AuCl3+3Br2 
Collection of gold2AuCl3+reducer→2Auo+6Cl−
Regeneration of hypohalides from barren solution6Cl−+electrooxidation→6OCl−6Br−+electrooxidation→6OBr−
In these various reactions, the anions, either halides or hypohalides, are carried by sodium as cations, i.e. NaCl, NaBr, NaOBr or NaOCl.
The implementation of this method involves the recycling of halogens, particularly the more expensive bromine. The liberation of the free halogens under acidic conditions generates an atmosphere rich in chlorine and bromine. The operation being conducted at atmospheric pressure, the excess halogens must be scrubbed, typically with sodium hydroxide NaOH, in order to recover these halogens for recycling. Vapors that accompany the filtration of the pregnant solution may also contain a small but significant amount of halogens that must be recovered for economic and environmental reasons. This is the first instance where addition of a chemical such as sodium hydroxide NaOH is required in the method.
A second instance of chemical addition in the method occurs at the time of collection of the gold dissolved in the pregnant solution, when a reducer is used to bring down the trivalent gold to an elemental state. This operation is achieved by reducing the Oxidation-Reduction Potential (ORP) from values in the range of 1000 mV to less than 400 mV vs a Ag/AgCl reference electrode. The reducer, either sodium sulfite Na2SO3 or sulfur dioxide gas SO2, involves sodium addition directly from the Na2SO3 or from the neutralization of the excess and oxidized SO2 (to H2SO4) with sodium hydroxide NaOH.
A third instance is after the gold collection, when the barren brine which has collected impurities, i.e. mainly iron, from the ore, must be purified before recycling by electrolysis. This purification is done by pH adjustment with sodium hydroxide in order to precipitate iron and other traces of base metals followed by a treatment with sodium carbonate Na2CO3 to bring down the calcium content to a level acceptable at the time of electrolysis.
Therefore at three steps in the above method, namely, at the time of scrubbing the off gases from the chlorination reactor, when precipitating the gold from the pregnant solution and at the time of the purification of the barren solution, the use of sodium hydroxide leads to an unavoidable accumulation of sodium. This sodium exists as sodium sulfate, since the acid used in the reactor is sulfuric acid H2SO and the reducer for gold precipitation is sulfur dioxide SO2 (see FIG. 1).
Removing sodium sulfate by crystallization is a known process but it is relatively expensive in capital and operation costs. Moreover, such a step eliminates the closed loop character of the method. Therefore, it was found highly desirable to improve upon these operational conditions that led to sodium build-up.
There is still a need in the art for a method and system for gold and silver extraction using halogens.